1. Field of the Invention
The present invention relates to a catalyst for olefin polymerization and a process for preparing olefin polymers therewith. More particularly, it relates to a catalyst for olefin polymerization which exhibits a high catalytic activity even in the absence of an expensive methylalumoxane and is capable of producing polymers having a good particle property on its application to the polymerization of olefins, and to a process for preparing olefin polymers therewith.
2. Background Art
There have recently been proposed highly active catalysts for olefin polymerization comprising a combination of a zirconocene compound and a methylalumoxane (Japanese Patent Laid-Open Publication Nos. 19309/1983 and 35007/1985). Also known are proposals which make possible to prepare a variety of stereospecific polymers by using zirconocene compounds of the structure of ligands specifically designed (Japanese Patent Laid-Open Publication Nos. 130314/1986, 295607/1988, 301704/1989 and 41303/1990). To the best of the knowledge of the present inventors, while it would be possible to prepare highly active polymers having a narrow molecular weight distribution by means of these proposals, polymers obtainable would only be those having a very small particle diameter (usually about 1-50 .mu.m) possibly because the catalysts may be soluble in the solvents used such as toluene and thus it would be difficult to produce polymers industrially in a high efficiency. These methods seem to require a large amount of an alumoxane and would thus be expensive, so that a variety of proposals have been done in order to solve the problems. It has been described in Japanese Patent Laid-Open Publication Nos. 51408/1989, 27560/1989 and 140305/1991 that an alumoxane and a metallocene compound are previously contacted to produce polymer products in a granulated form. However, to the best of our knowledge, the granulation of the polymer with this method would be possible only in the so-called liquid-phase bulk polymerization with propylene as a solvent, and no polymer having a controlled particle diameter would be obtained by slurry polymerization with an inert solvent or vapor phase polymerization.
Catalysts in which a catalyst ingredient is supported on an inorganic compound, particularly silica, have been proposed in Japanese Patent Laid-Open Publication Nos. 296008/1986, 51407/1988 and 152608/1988 and in WO 88/05058.
It has also been proposed to support a catalyst ingredient on polyethylene particles in Japanese Patent Laid-Open Publication No. 92621/1988 and WO 88/05058. Though these techniques would have made it possible to produce a granular polymer even in the case of vapor phase polymerization, problems would still remain such as poor polymer properties due to the content of a support material in the product which may amount to a considerable level because of insufficient catalyst activity per support which requires considerable amount of the catalyst plus support or the consumption of an expensive alumoxane in a large amount.
On the other hand, Japanese Patent Laid-Open Publication Nos. 501950/1989 and 502036/1989 and Japanese Patent Laid-Open Publication Nos. 179005/1991, 207703/1991 and 207704/1991 disclose catalysts comprising a Lewis acid such as an organoboron compound, for example tris(pentafluorophenyl)boron, an anionic compound such as carborane and tetrakis(pentafluorophenyl)borate, and a combination of an organoboron compound and a trialkylaluminum. However, polymerization with these catalysts would produce polymers only in the form of very small particles and commercial production of polymers in a high efficiency would not be feasible. Also, Japanese Patent Laid-Open Publication Nos. 179005/1991, 142306/1992, 148316/1993, 239138/1993 and 247128/1993 disclose catalyst systems comprising a supported organoboron co-catalyst, but the catalysts comprising these supported organoboron compounds, which include no boric acid, would, to the best of the knowledge of the present inventors, suffer from the catalyst activity appreciably lowered as compared with catalysts comprising no such supported organoboron compounds whereby they exhibit little cost cut effect in spite of employing no methylalumoxanes, and, in addition, the catalyst activity per support would be at an insufficient level. It has thus been desired to improve the catalyst systems.
As the other techniques for preparing catalysts for olefin polymerization, the combinations of a water-containing inorganic compound and an organoaluminum compound are described in Japanese Patent Laid-Open Publication Nos. 207303/1989, 224808/1992, and Japanese Patent Laid-Open Publication No. 501424/1993, but none of these techniques seem to us to give satisfactory catalyst activity. The employment of a Broensted acid or a clay mineral is described in Japanese Patent Laid-Open Publication No. 157651/1994 and in Japanese Patent Laid-Open Publication Nos. 105721/1993 and 301917/1993, respectively, but the latter proposal would entail a problem that the clay mineral is exceptional as a support and it would thus difficult to control its characteristics.
The present invention has been done in order to solve the problems in the prior art, and the objects of the present invention is to provide a catalyst which makes possible to prepare a polymer having a good particle property in a high yield even in the absence of an expensive alumoxane even if it is applied to slurry polymerization or vapor phase polymerization without any special support material and to prepare an olefin polymer with the catalyst.